Expander

ABSTRACT

An expander includes a piston formed from a top portion, a skirt portion, and a land portion. A hollow heat-insulating space is formed within the piston making heat transfer difficult. It is possible to maintain the land portion, which is in contact with the high temperature, high pressure steam in an expansion chamber, at a high temperature, thereby minimizing any decrease in the temperature of the high temperature, high pressure steam to prevent any decrease in the efficiency of the expander. Any increase in the temperature of the skirt portion, which is in sliding contact with a cylinder sleeve, is suppressed to ensure the performance of the lubrication. Piston rings are provided on the land portion to separate the high temperature, high pressure steam in the expansion chamber from the oil in the skirt portion for preventing the oil and the high temperature, high pressure steam from being mixed together.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2003-392757 filed on Nov. 21, 2003 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an expander that includes a casing, a rotor rotatably supported in the casing, and an axial piston cylinder group arranged in the rotor so as to surround an axis of the rotor. The rotor is rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group. Sliding sections of the piston and the cylinder sleeve are lubricated with oil.

2. Description of Related Art

An expander is known as disclosed in Japanese Patent Application Laid-open No. 2002-256805. This expander includes a first axial piston cylinder group arranged on the radially inner side and a second axial piston cylinder group arranged on the radially outer side. A piston of the first axial piston cylinder group has a solid structure with one end facing an expansion chamber, to which high temperature, high pressure steam is supplied. The other end abuts against a swash plate, while sliding sections of the piston and a cylinder sleeve are lubricated with oil.

In a piston of an axial piston cylinder group of an expander, it is desirable that an end thereof facing an expansion chamber is maintained at a high temperature so that the high temperature, high pressure steam supplied to the expansion chamber does not decrease. However, it is desirable for sliding sections of the piston and a cylinder sleeve to be maintained at a low temperature so as to ensure proper lubrication. However, if the piston has a solid structure, heat is quickly transmitted from the high temperature side to the low temperature side, so that the temperature of the end on the expansion chamber side, which should be maintained at a high temperature, easily decreases. In addition, the temperature of the sliding sections of the piston and the cylinder sleeve, which should be maintained at a low temperature, easily increases.

SUMMARY OF THE INVENTION

The present invention has been developed to eliminate the above-mentioned circumstances. It is an object of the present invention to suppress the escape, via a piston, of the heat from the high temperature, high pressure steam supplied to an expansion chamber of an axial piston cylinder group of an expander, for ensuring that the lubrication performance is in sliding sections of the piston and a cylinder sleeve.

In order to achieve the above-mentioned object, in accordance with a first aspect of the present invention, there is proposed an expander that includes a casing, a rotor rotatably supported in the casing, and an axial piston cylinder group arranged in the rotor so as to surround an axis of the rotor. The rotor is rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group, with sliding sections of the piston and the cylinder sleeve being lubricated with oil. The piston of the axial piston cylinder group has a land portion that is exposed to the high temperature, high pressure steam in the expansion chamber with a top portion that abuts against a swash plate, and a skirt portion that is disposed between the land portion and the top portion and slides against the cylinder sleeve. The piston has a hollow heat-insulating space formed therewithin. The land portion has a piston ring provided thereon with the piston ring separating the high temperature, high pressure steam in the expansion chamber from oil in the skirt portion.

Furthermore, in accordance with a second aspect of the present invention, a plurality of piston rings are provided along the longitudinal direction of the piston.

Moreover, in accordance with a third aspect of the present invention, a depression is provided between the plurality of piston rings.

Furthermore, in accordance with a fourth aspect of the present invention, an oil channel is provided in at least one of an outer peripheral face of the skirt portion of the piston and an inner peripheral face of the cylinder sleeve.

A second land channel 63 e of an embodiment corresponds to the depression of the present invention, and a top ring 65 and a second ring 66 of the embodiment correspond to the piston rings of the present invention.

In accordance with the arrangement of the first aspect, the piston of the axial piston cylinder group is formed from the land portion exposed to the high temperature, high pressure steam in the expansion chamber with the top portion abutting against the swash plate, and the skirt portion disposed between the land portion and the top portion and sliding against the cylinder sleeve. A hollow heat-insulating space is formed therewithin to make heat transfer difficult. Therefore, it is possible to maintain the land portion, which is in contact with the high temperature, high pressure steam supplied to the expansion chamber, at a high temperature, thereby minimizing any decrease in the temperature of the high temperature, high pressure steam to prevent any decrease in efficiency of the expander. Also, it is possible to suppress any increase in the temperature of the skirt portion, which is in sliding contact with the cylinder sleeve, to ensure lubrication performance. Moreover, since the piston ring provided on the land portion separates the high temperature, high pressure steam in the expansion chamber from the oil in the skirt portion, it is possible to prevent the oil from entering the expansion chamber side and cooling the land portion, and to prevent the high temperature, high pressure steam from entering the skirt portion side and degrading the lubricating effect of the oil.

In accordance with the arrangement of the second aspect, since the plurality of piston rings are provided along the longitudinal direction of the piston, not only is it possible and more reliably to prevent the high temperature, high pressure steam from blowing past the land portion to the skirt portion, it is also possible to effectively prevent the oil and the high temperature, high pressure steam from mixing together by imparting the function of an oil ring to the piston ring which is on the side far from the expansion chamber.

In accordance with the arrangement of the third aspect, the depression is provided between the plurality of piston rings. Therefore, even if some of the high temperature, high pressure steam blows past the piston ring on the expansion chamber side, it is possible to prevent, by the effect of the volume of the depression, a pressure difference between the expansion chamber side and the depression side of the piston ring from abruptly decreasing, to thus urge radially outwardly the piston ring by the pressure difference to prevent the piston ring from floating above an inner peripheral face of the cylinder sleeve or a piston ring channel face, thereby ensuring sealing performance.

In accordance with the arrangement of the fourth aspect, since the oil channel is provided in at least one of the outer peripheral face of the skirt portion of the piston and the inner peripheral face of the cylinder sleeve, it is possible to retain oil in this oil channel to enhance the lubrication performance between the sliding surfaces of the piston and the cylinder sleeve.

A mode for carrying out the present invention is explained below with reference to an embodiment of the present invention shown in the attached drawings.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a vertical sectional view of an expander;

FIG. 2 is a sectional view along line 2-2 in FIG. 1;

FIG. 3 is a view taken along line 3-3 in FIG. 1;

FIG. 4 is an enlarged view of part 4 in FIG. 1;

FIG. 5 is an exploded perspective view of a rotor;

FIG. 6 is a sectional view along line 6-6 in FIG. 4;

FIG. 7 is a sectional view along line 7-7 in FIG. 4; and

FIG. 8 is an enlarged view of part 8 in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An expander E of this embodiment is used in, for example, a Rankine cycle system. The expander E converts the thermal energy and the pressure energy of the high-temperature, high-pressure steam as a working medium into mechanical energy, and outputs the energy. A casing 11 of the expander E is formed from a casing main body 12, a front cover 15 joined via a seal 13 to a front opening of the casing main body 12 by a plurality of bolts 14, a rear cover 18 joined via a seal 16 to a rear opening of the casing main body 12 by a plurality of bolts 17, and an oil pan 21 joined via a seal 19 to a lower opening of the casing main body 12 by a plurality of bolts 20.

A rotor 22 is arranged rotatably around an axis L extending in the fore-and-aft direction through the center of the casing 11. The rotor 22 includes a front part supported by combined angular bearings 23 provided in the front cover 15, and a rear part thereof supported by a radial bearing 24 provided in the casing main body 12. A swash plate holder 28 is formed integrally with a rear face of the front cover 15. A swash plate 31 is rotatably supported by the swash plate holder 28 via an angular bearing 30. The axis of the swash plate 31 is inclined relative to the axis L of the rotor 22, and the angle of inclination is fixed.

The rotor 22 includes an output shaft 32 supported in the front cover 15 by the combined angular bearings 23 with three sleeve support flanges 33, 34, and 35 being formed integrally with a rear part of the output shaft 32 via cutouts 57 and 58 having predetermined widths (see FIG. 4 and FIG. 8). A rotor head 38 is joined by a plurality of bolts 37 to the rear sleeve support flange 35 via a metal gasket 36 and is supported in the casing main body 12 by the radial bearing 24. A heat-insulating cover 40 is fitted over the three sleeve support flanges 33, 34, and 35 from the front and joined to the front sleeve support flange 33 by a plurality of bolts 39.

Sets of five sleeve support holes 33 a, 34 a, and 35 a are formed in the three sleeve support flanges 33, 34, and 35 respectively at intervals of 72° around the axis L. Five cylinder sleeves 41 are fitted into the sleeve support holes 33 a, 34 a, and 35 a from the rear. A flange 41 a is formed on the rear end of each of the cylinder sleeves 41. An axial positioning is carried out by abutting the flange 41 a against the metal gasket 36 while fitting the flange 41 a into a step 35 b formed in the sleeve support holes 35 a of the rear sleeve support flange 35 (see FIG. 8). A piston 42 is slidably fitted within each of the cylinder sleeves 41 with the front end of the piston 42 abutting against a dimple 31 a formed on the swash plate 31. A steam expansion chamber 43 is defined between the rear end of the piston 42 and the rotor head 38.

A plate-shaped bearing holder 92 is superimposed on a front face of the front cover 15 via a seal 91 and is fixed thereto by means of bolts 93. A pump body 95 is superimposed on a front face of the bearing holder 92 via a seal 94 and is fixed thereto by means of bolts 96. The combined angular bearings 23 are held between a step of the front cover 15 and the bearing holder 92, thereby fixing them in the axis L direction.

A shim 97 having a predetermined thickness is held between the inner race of the combined angular bearings 23 and a flange 32 d formed on the output shaft 32 supporting the combined angular bearings 23. The inner race of the combined angular bearings 23 is tightened by a nut 98 screwed around the outer periphery of the output shaft 32. As a result, the output shaft 32 is positioned in the axis L direction relative to the combined angular bearings 23, that is, relative to the casing 11.

An oil passage 32 a is formed so as to extend along the axis L within the output shaft 32 which is integral with the rotor 22. The front end of the oil passage 32 a branches in a radial direction and communicates with an annular channel 32 b on the outer periphery of the output shaft 32. An oil passage blocking member 45 is screwed into the inner periphery of the oil passage 32 a via a seal 44 at a position that is radially inside the middle sleeve support flange 34 of the rotor 22. A plurality of oil holes 32 c extend radially outwardly from the oil passage 32 a in the vicinity of the oil passage blocking member 45 and open on the outer periphery of the output shaft 32.

A trochoidal oil pump 49 is disposed between a recess 95 a provided in a front face of the pump body 95 and a pump cover 48 fixed via a seal 46 to the front face of the pump body 95 by a plurality of bolts 47. The trochoidal oil pump 49 includes an outer rotor 50 that is rotatably fitted in the recess 95 a, and an inner rotor 51 that is fixed to the outer periphery of the output shaft 32 and meshes with the outer rotor 50. An internal space of the oil pan 21 communicates with an intake port 53 of the oil pump 49 via an oil pipe 52 and an oil passage 95 b of the pump body 95. A discharge port 54 of the oil pump 49 communicates with the annular channel 32 b of the output shaft 32 via an oil passage 95 c of the pump body 95.

The structure of the piston 42 is now explained in detail with reference to FIG. 5 to FIG. 8.

The piston 42 is formed from a top portion 61, a skirt portion 62, and a land portion 63. The top portion 61 is a member having a spherical portion 61 a that abuts against a dimple 31 a of the swash plate 31, and is joined to the front end of the skirt portion 62 by welding. The land portion 63 and the skirt portion 62 are formed integrally and have a large volume with an evacuated, heat-insulating space 64 defined therewithin. An annular oil channel 62 a is formed by slightly decreasing the diameter in a middle section of the skirt portion 62 which is slidably fitted into an inner peripheral face of the cylinder sleeve 41. A plurality of spiral channels 62 b are formed in an outer peripheral section of the skirt portion 62 on the front side relative to the oil channel 62 a.

The land portion 63 has a top land 63 a on the expansion chamber 43 side and a second land 63 b on the skirt portion 62 side. A top ring channel 63 c is formed between the top land 63 a and the second land 63 b with a top ring 65 being fitted around the top ring channel 63 c. A second ring channel 63 d is formed between the second land 63 b and the skirt portion 62 with a second ring 66 being fitted around the second ring channel 63 d. A second land channel 63 e is formed in a middle section of the second land 63 b. The top land 63 a and the second land 63 b have an outer diameter that is slightly smaller than the outer diameter of the skirt portion 62. A gap a is formed between outer peripheral faces of the top land 63 a and the second land 63 b and the inner peripheral face of the cylinder sleeve 41. Therefore, a thrust load in the peripheral direction for rotating the rotor 22 is transmitted from the skirt portion 62 to the cylinder sleeve 41 without passing through the land portion 63.

A top ring 65, which is disposed at a position comparatively close to the end of the piston 42 (on the order of 10% of the diameter of the piston 42), has a rectangular cross-section with a face that is in sliding contact with the cylinder sleeve 41 which is a curved barrel face. A hard coating of a ceramic such as TiN or CrN is formed on the surface thereof. The second ring 66 has a rectangular cross-section (or an internal bevel cut cross-section); a face that is in sliding contact with the cylinder sleeve 41 which is a curved barrel face. A hard coating of a ceramic such as TiN or CrN is formed on the surface thereof. For both the top ring 65 and the second ring 66, the gap across the ends is set to be the minimum gap at which the ends do not make contact when hot with the thickness being set as small as possible (on the order of 3% of the diameter of the piston 42), and the initial tension being set fairly small.

Applying such a hard coating to the top ring 65 and the second ring 66 decreases the amount of wear and the amount of leakage of steam to be reduced. Since the initial tension of the top ring 65 and the second ring 66 is set to be small and the thickness is set to be small so that the load pushing the inner peripheral face of the cylinder sleeve 41 due to the pressure of the steam is reduced, it is possible to reduce the frictional force between the cylinder sleeve 41 and the top ring 65 and the second ring 66, while enhancing the sealing effect by improving the ability of the top ring 65 and the second ring 66 to follow the cylinder sleeve 41.

An annular channel 41 b is formed on the outer periphery of a middle part of the cylinder sleeve 41 (see FIG. 5 and FIG. 8), and a plurality of oil holes 41 c are formed in the annular channel 41 b. The oil channel 62 a formed in the middle section of the skirt portion 62 communicates with the oil holes 41 c of the cylinder sleeve 41.

An annular cover member 69 is welded to the front side or the expansion chamber 43 side of the rotor head 38 which is joined to the rear face of the rear sleeve support flange 35 of the rotor 22 by the bolts 37. Thus, an annular heat-insulating space 70 is defined at the back face or rear face of the cover member 69 (see FIG. 8). The rotor head 38 is positioned rotationally relative to the rear sleeve support flange 35 by a knock pin 55.

As shown in FIG. 1, a rotary valve 71 is provided between the rear cover 18 of the casing 11 and the rotor head 38 of the rotor 22. The rotary valve 71 supplies the high temperature, high pressure steam from a steam supply pipe 67 sequentially to five expansion chambers 43, accompanying rotation of the rotor 22, and discharges a low temperature, low pressure steam from the expansion chambers 43 to a steam discharge chamber 68 formed between the main body casing 12 and the rear cover 18.

The five cylinder sleeves 41 and the five pistons 42 form an axial piston cylinder group A of the present invention.

The operation of the expander E of this embodiment having the above-mentioned arrangement is now explained.

When the high temperature, high pressure steam generated by heating water in an evaporator that is supplied from the steam supply pipe 85 via the rotary valve 71 to the expansion chamber 43 within the cylinder sleeve 41, the piston 42 fitted in the cylinder sleeve 41 is pushed forward from a top dead center toward a bottom dead center, so that the top portion 61 at the front end of the piston 42 pushes against the dimple 31 a of the swash plate 31. As a result, the reaction force that the pistons 42 receive from the swash plate 31 gives a rotational torque to the rotor 22. For each one fifth of a revolution of the rotor 22, the high-temperature, high-pressure steam is supplied into a fresh adjoining expansion chamber 43, thus continuously rotating the rotor 22. While the piston 42, having reached the bottom dead center accompanying the rotation of the rotor 22, retreats toward the top dead center by being pushed by the swash plate 31, the low-temperature, low-pressure steam pushed out of the expansion chamber 43 is discharged into the steam discharge chamber 68 via the rotary valve 71.

The oil pump 49 provided on the output shaft 32 operates together with the rotation of the rotor 22. Oil is taken in from the oil pan 21 via the oil pipe 52, the oil passage 95 b of the pump body 95, and the intake port 53 and is discharged from the discharge port 54, and supplied to the oil channel 62 a formed in the skirt portion 62 of the piston 42 via the oil passage 95 c of the pump body 95, the oil passage 32 a of the output shaft 32, the annular channel 32 b of the output shaft 32, the oil holes 32 c of the output shaft 32, the annular channel 41 b of the cylinder sleeve 41, and the oil holes 41 c of the cylinder sleeve 41. A portion of the oil retained by the oil channel 62 b flows into the spiral oil channels 62 b formed in the skirt portion 62 of the piston 42, lubricates the surface that slides against the cylinder sleeve 41, and is then returned to the oil pan 21. Another portion of the oil lubricates the surfaces of the top ring 65 and the second ring 66 that slide against the cylinder sleeve 41, the top ring 65 and the second ring 66 are provided in the land portion 63 of the piston 42. Since the oil channel 62 a formed in the skirt portion 62 has the function of temporarily retaining oil, it is possible to continuously supply oil to the sliding sections of the piston 42 and the cylinder sleeve 41, thus improving the lubrication conditions.

Since the evacuated, heat-insulating space 64 is formed in the interior of the piston 42, it is possible to suppress the escape, via the piston 42, of the heat of high temperature, high pressure steam supplied to the expansion chamber 43 which faces the end of the piston 42, thus minimizing any decrease in the temperature of the high temperature, high pressure steam in the expansion chamber 43 to increase the output of the expander E. Furthermore, since the top of the piston 42 is maintained at a high temperature, it becomes difficult for the steam to condense and liquify between the land portion 63 of the piston 42 and the cylinder sleeve 41, thus improving the lubrication conditions of the land portion 63 to improve the sealing performance and the wear resistance of the top ring 65 and the second ring 66.

Since the rear side of the top ring channel 63 c of the land portion 63 of the piston 42 communicates with the expansion chamber 43 which is at a high pressure, and the front side thereof communicates with the second land channel 63 e which is at a low pressure, the difference in pressure pushes the top ring 65 from the bottom part of the top ring channel 63 c to make the top ring 65 come into intimate contact with the inner peripheral face of the cylinder sleeve 41 and the side face of the top ring channel 63 c, thereby improving the sealing performance. Even if a part of the high temperature, high pressure steam in the expansion chamber 43 blows past the top ring 65 toward the second land channel 63 e, it is possible to suppress an abrupt decrease in the difference in pressure by virtue of the volume of the second land channel 63 e, thus maintaining the tension of the top ring 65 to prevent the sealing performance from deteriorating. Moreover, the second land channel 63 e also has the function of retaining oil, thus contributing to an improvement in the lubrication performance.

The second ring 66 maintains the compression when the high temperature, high pressure steam blows past the top ring 65, and has the function of an oil ring to scrape off oil attached to the inner peripheral face of the cylinder sleeve 41. In this way, the high temperature, high pressure steam in the expansion chamber 43 is separated from the oil in the skirt portion 62 by means of the top ring 65 and the second ring 66 provided on the land portion 63, to thereby prevent the oil from entering the expansion chamber 43 side and cooling the land portion 63. Thus, the high temperature, high pressure steam is prevented from entering the skirt portion 62 side and degrading the lubricating effect of the oil.

Although an embodiment of the present invention is explained above, the present invention can be modified in a variety of ways without departing from the subject matter thereof.

For example, the axial piston cylinder group A of the embodiment includes the five pistons 42 and the five cylinder sleeves 41, but the numbers thereof are not limited to those of the embodiment.

Furthermore, the heat-insulating space 64 of the embodiment is evacuated. However, the heat-insulting space 64 may have a gas such as air sealed inside.

Moreover, in the embodiment, the oil channel 62 a is provided in the outer peripheral face of the piston 42, but an oil channel may be provided in the inner peripheral face of the cylinder sleeve 41.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A piston for use in a rotor comprising: an axial piston cylinder group arranged in the rotor for surrounding an axis of the rotor, said rotor being adapted to be rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group with sliding sections of the piston and the cylinder sleeve being lubricated with oil; a land portion formed on said piston of the axial piston cylinder group, said land portion being exposed to the high temperature, high pressure steam in the expansion chamber; a top portion formed on said piston adapted to abut against a swash plate; a skirt portion formed on said piston that is disposed between the land portion and the top portion for sliding against the cylinder sleeve; a hollow heat-insulating space formed within the piston; and a piston ring provided on the land portion, the piston ring separating high temperature, high pressure steam in the expansion chamber from oil in the skirt portion.
 2. The piston for use in a rotor according to claim 1, wherein a plurality of piston rings are provided along the longitudinal direction of the piston.
 3. The piston for use in a rotor according to claim 2, wherein a depression is provided between the plurality of piston rings.
 4. The piston for use in a rotor according to claim 1, wherein an oil channel is provided in at least one of an outer peripheral face of the skirt portion of the piston and an inner peripheral face of the cylinder sleeve.
 5. The piston for use in a rotor according to claim 1, wherein the top portion includes a spherical portion adapted to abut against the swash plate.
 6. The piston for use in a rotor according to claim 1, wherein the heat-insulating space is evacuated.
 7. The piston for use in a rotor according to claim 1, wherein the heat-insulating space includes air sealed therein.
 8. The piston for use in a rotor according to claim 1, wherein a ceramic selected from the group consisting of TiN and CrN is formed on an outer surface of said piston ring.
 9. The piston for use in a rotor according to claim 1, and further including an oil channel formed in a mid-section of the skirt portion of said piston.
 10. The piston for use in a rotor according to claim 9, wherein a plurality of spiral oil channels are formed in the mid-section of the skirt portion of said piston.
 11. An expander comprising: a casing; a rotor rotatably supported in the casing; and an axial piston cylinder group arranged in the rotor so as to surround an axis of the rotor; the rotor being rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group with sliding sections of the piston and the cylinder sleeve being lubricated with oil; said piston of the axial piston cylinder group including a land portion that is exposed to the high temperature, high pressure steam in the expansion chamber, a top portion that abuts against a swash plate, and a skirt portion that is disposed between the land portion and the top portion and slides against the cylinder sleeve; a hollow heat-insulating space formed within the piston; and a piston ring provided on the land portion, the piston ring separating high temperature, high pressure steam in the expansion chamber from oil in the skirt portion.
 12. The expander according to claim 11, wherein a plurality of piston rings are provided along the longitudinal direction of the piston.
 13. The expander according to claim 12, wherein a depression is provided between the plurality of piston rings.
 14. The expander according to claim 11, wherein an oil channel is provided in at least one of an outer peripheral face of the skirt portion of the piston and an inner peripheral face of the cylinder sleeve.
 15. The expander according to claim 11, wherein the top portion includes a spherical portion for abutting against the swash plate.
 16. The expander according to claim 11, wherein the heat-insulating space is evacuated.
 17. The expander according to claim 11, wherein the heat-insulating space includes a air sealed therein.
 18. The expander according to claim 11, wherein a ceramic selected from the group consisting of TiN and CrN is formed on an outer surface of said piston ring.
 19. The expander according to claim 11, and further including an oil channel formed in a mid-section of the skirt portion of said piston.
 20. The expander according to claim 19, wherein a plurality of spiral oil channels are formed in the mid-section of the skirt portion of said piston. 